Hydraulic axial piston machine of the bent-axis type

ABSTRACT

A hydraulic or pneumatic so called bent-axis piston machine is provided which comprises a number of parallel cylinders arranged equally spaced about a common axis, each cylinder slidably and telescopically receiving a piston in the form of a sleeve forming together with its cylinder a piston-cylinder assembly whose opposite ends engage spherical convex seats provided on respective rotatable support bodies or drive plates, said bodies or plates forming an angle with one another in the manner known in bent-axis machines. Each cylinder and piston is resiliently held in engagement with its cooperating seat by means of a spring-actuated expander device tending to bring each of the piston-cylinder assemblies to extend in order that their seating engagement is maintained. One of the support bodies or drive plates may also be made adjustably slidable in a lateral direction so as to vary the bent-axis angle and thus the displacement of the machine.

United States Patent [191 Freese Mar. 27, 1973 [54] HYDRAULIC AXIAL PISTON MACHINE OF THE BENT-AXIS TYPE Primary Examinen-William L. Freeh [75] Inventor: Lennart Werner Freese, Jarfalla, Ana 3'', Blanchard and Flynn Swede [57 ABSTRACT [73] Asslgnee: General Electric Company A hydraulic or pneumatic so called bent-axis piston [22] Filed: Mar. 26, 1971 machine is provided which comprises a number of parallel cylinders arranged equally spaced about a [2n Appl' common axis, each cylinder slidalbly and telescopically receiving a piston in the form of a sleeve forming [30] Foreign Application Priofity Dal; together with its cylinder a piston-cylinder assembly whose opposite ends engage spherical convex seats Apr. 3,1970 Sweden ..4598/70 on respective rotatable pp bodies or drive plates. said. bodies or plates forming an angle [52] US. Cl....., ..9l/504 h one another in the manner known in benbwds [51] Ill!- ..F01b machinea Each cylinder and piston i resilicnfly d 58] Field Of Search ..91/49(), 1-99, 505 in engagement with its coopcrating Seat by means of a spring-actuated expander device tending to bring each [56] References Cited of the piston-cylinder assemblies to extend in order HTS that their seating engagement is maintained. One of UNITED STATES PATE the support bodies or drive plates may also be made 2,146,133 2/1939 Tweedale ..91/505 adjustably slidable in a lateral direction so as to vary 2,967,491 1/1961 Wiggermann.... ..91/505 the bent-axis angle and thus the displacement of the 3,108,543 10/1963 Gregor ..9l/490 machine, 3,123,014 3/1964 Gregor..... ..91/490 1 3,220,353 11/1965 Gregor ..91 490 8 Claims, 4 Drawing Figures guy, "2 12 10 Y; 3 11; 21. 20 Q F 22 \i:

21 in U 92 23 -57 [.0 9B 'l' Q /-'r v.

The present invention relates to hydraulic axial piston machines, and more specifically to the type designated bent-axis machines. The invention also relates to machines of this type which work with gaseous media, such as pneumatic machines.

A common feature of the hydraulic axial piston machines is that they are provided with a number of pistons slidably accomodated in corresponding cylinders formed in a rotatably joumalled drum-like body and disposed in uniform spaced relationship arouiid and substantially parallel with the axis of rotation of the drum. The pistons are either connected by means of piston rods with a drive plate on the drive shaft of the machine, or the free ends of the pistons are caused to abut, via a ball joint, against piston shoes which are capable of sliding against a non-rotatable camplate accomodated in the machine housing. The former type is designated a bent-axis machine and the relative movement between pistons and cylinders in this type of machine is provided for by the fact that during rotation the center lines of the cylinder drum and the drive shaft form a specific angle with each other, while with the second type of machine, which is called an in-line machine, the aforementioned relative movement is provided for by the fact that the cam plate is positioned obliquely in relation to the cylinder drum, which is in this case coaxial with its drive shaft. Both of these machine types have characteristic advantagesand disadvantages.

. It has been proposed to use the telescopic pistoncylinder assembly described in my co-pending application Ser. No. 114,935, filed Feb. 12, 1971, as a working element in a hydraulic axial piston machine, preferably the bent-axis type. In this way a number of advantages can be gained over similar, conventional types of machines, particularly with respect to the saving of space and simplified manufacture. This new machine, however, also has a number of disadvantages which are common to existing conventional machines, and the object of the invention is therefore to provide an improved hydraulic bent-axis machine having a telescopic piston-cylinder assembly and which is superior, both with respect to manufacture and to construction, to conventional machines of the in-line as well as the bent-axis type. This object is achieved and the hydraulic bent-axis machine of small external dimensions and low weight in relation to its maximum effect, and which can be made an invariable or a variable displacement machine, is obtained with the machine having the characteristic features disclosed in claim 1.

A number of embodiments of the invention will how be described by way of example, with reference to the accompanying drawings, in which FIG. 1 is a view of a central longitudinal section through a positive displacement hydraulic bent-axis piston machine according to the invention, while FIG. 2 is a diagrammatic cross section of the ring of piston-cylinder assemblies incorporatediin the machine of FIG. 1 and is intended to illustrate an entraining driving function. FIG. 3 is a part sectional view'corresponding substantially to FIG. 1 but showing, somewhat diagrammatically, a variable displacement machine according to the invention, and finally FIG. 4 is a sectional view taken along the line lV'--lV in FIG; 3.

As will be evident from FIG. 1 the piston-cylinder assemblies used in the bent-axis machine of the present invention coincide substantially with those described in the aforementioned application. Thus, each assembly includes a sleeve-shaped piston 12 which is slidably accomodated in a similarly sleeve-shaped cylinder 10, the two elements being mutually sealed by means of a sealing ring 13, Both piston and cylinder engage at their opposing outer ends spherical seating surfaces 24 and 22, v

The peg-like bodies 20 with which the pistons 12 engage are made integral with or are fixedly attached to a guide plate :40 mounted for rotation in a cover portion 420 of the machine housing 42, for example by being joined with a central pin 32 accomodated in a bearing housing 33 arranged in the wall of the housing. The opposing seating bodies 18, with which the ends of cylinder 10 engage, are made integral with or are fixedly attached to a drive plate 44 which is non-rotatably connected with the driven or driving shaft 46 of the machine, for example by' being wedged fixedly to the shaft or by being made integral therewith in the form of a flange. The drive plate 44 may either by rotatably mounted in a cover portion 42b of the machine housing 42, by'means of a roller bearing 48 for example, or the radial forces acting on the drive plate may be taken up by a bearing 50 of the drive shaft 46.

The necessary relative movement between pistons and'cylinders upon rotation of the shaft is provided for by the fact that the axis of rotation 52 of the guide plate 40 forms a certain anglea with the axis of rotation 54 of the drive shaft. The two axes of rotation intersect each other at or in the proximity of the centers of the spherical seating surfaces 22.

Working fluid is passed to and from the pistoncylinder assemblies through valve ports 56 and 56 arranged in the cover 42b of the machine housing. The Ports 56 and 56 are so shaped (kidney shaped) that during rotation of the driving shaft the interior of the piston-cylinder assemblies is alternately placed in communication with, the supply and. return lines of the hydraulic system via connecting .passages 58 and 58,

respectively, connection between the interior of the assemblies and the ports taking place through coaxial passages 26 and 28 arranged in the seating bodies 18 and the drive plate 44, respectively. Thus, the flow of working fluid to and from the piston-cylinder assemblies takes place in a manner conventional with hydraulic axial piston machines and therefore need not be described in more detail.

An important feature of the bent-axis machine of the present'invention resides in a combined arrangement for driving the opposing guide plate 40 and for reducing the dead space or volume in the piston-cylinder assembly, i.e. the fluid accommodating volume of the assembly in excess of the stroke volume. This'combined arrangement for driving the guide plate 40 and reducing dead space co-acts with the seating bodies 20 of the guide plate 40, since the seating bodies are provided with axial, cylindrical or conical extensions or pins 23 which occupy to the greatest possible extent the space in the interior of the piston-cylinder assemblies. It will be immediately understood that because of this arrangement the elastic losses caused by the combination of the dead space and the compressibility of the working fluid and which are especially troublesome in high pressure machines are substantially reduced. Naturally in this respect the pins 23 should be of such configuration that they do not prevent the pendulating movement relative to the seating surfaces 24 which the piston-cylinder assemblies must execute during rotation of the machine and which is caused by the kinematic conditions of the machine.

On the other hand, the pins 23 are so constructed that subsequent to permitting the necessary angular movement of the pistons 12 relative the seating surfaces 24 they make contact with the interior of the pistons and function as drive dogs. This is illustrated in more detail in FIG. 2, which shows a cross section through the complete ring of piston-cylinder assemblies and the pins 23 extending thereinto. When the drive shaft 46 begins to rotate, the shaft and the guide plate 40 are at first twisted a little relative to each other, until each pin 23 abuts the interior of its associated piston 12 at a point or along a line 30. The plate 40 is in this way caused to rotate synchronously with the drive shaft and the pins 23 thus replace other, more complicated devices for driving the plate 40, for example such devices as universal joints used with known machines of the type in question.

. In order for a hydraulic machine equipped with the above described piston-cylinder assemblies 12, to operate smoothly, it is necessary that the piston and cylinder of each assembly is constantly held in sealing engagement with its cooperating seating on the seating bodies and 18 respectively. The most obvious way to accomplish this is to embody pressure springs and to arrange the springs or each assembly, either externally or internally, and to cause the piston and cylinder to be forced apart, as described in the aforementioned patent. A considerably improved arrangement for moving the piston and cylinder of each assembly apart and holding them in engagement with their respective seating surfaces is proposed, however, in accordance with the invention. This arrangement includes a circular plate 72, see FIGS. 1 and 2, having a diameter slightly larger than the full ring of piston-cylinder assemblies, there being arranged in the plate openings or recesses 74 which correspond in number, shape and position to the assembles so that the latter can pass through their respective openings in the plate. The plate engages, at a non-fixed point on the edge of each opening, with a shoulder or flange 76 located on the outside of cylinder 10 passing through respective openings, so that an axial force directed towards the seating surfaces of the cylinders can be transmitted from the plate 72 to the cylinders to assist in holding the same against their respective seating surfaces. The shape of the openings 74 is such that the openings permit certain mutual movement between the piston-cylinder assemblies and an oblique positioning of the assemblies relative the circular plate 72. Analogously, a second plate 78 engages shoulders or flanges located on the pistons 12 and subjects the pistons to an axial force, thereby holding the same against their respective seating surfaces 24. Owing to the fact that mutual relative movement of the pistons is slight, the plate 78 need not fully encircle the piston-cylinder assemblies, but need only be arranged to engage constantly respectively piston at practically one and the same point 82 located nearest the common axis of rotation 52 of the assemblies.

In order to produce the mutually opposing axial forces on the plates 72 and 78 necessary for holding the pistons and cylinders apart, a coil spring 84 is conveniently arranged between the plates concentrical with the axis of rotation 52. The spring is arranged so that one end thereof bears against the plate 78 (FIG. 1) while its other end bears against a spring seating 86 which is in the form of a sleeve slidably arranged on an extension of the journal pin 32. The sleeve is provided with a curved surface 88 which when rolling along the plate 72 subjects the plate to a resilient force which is distributed on the cylinders 10 of the piston-cylinder assemblies, as clearly shown in FIG. 1.

The above description of the first embodiment of the hydraulic angle piston machine of the present invention will be terminated with a discussion of the possibilities of hydraulically balancing the machine. As is well known, the large axial forces which are developed and transmitted reactionwise from the rotating components of hydraulic piston machines to the stationary housing of the machine while carrying out useful work constitute a serious problem of construction with all hydraulic piston machines. In order to remove these difficulties, the expedient of hydraulic balancing is employed to a large extent, i.e. the forces which occur are balanced against each other and transmitted to the surroundings hydraulically, so that the least and lightest possible direct mechanical contact is obtained between stationary and movable components. It has been found that these large forces can be substantially or almost completely balanced out hydraulically with the bentaxis machine of the present invention (FIGS. 1 and 3). When considering the embodiment of FIG. 1, it will be seen that the force system developed by the ringof piston-cylinder assemblies 12, 10 is primarily taken up, in one hand, by the drive plate 44, to the right, and, on the other, by the guide plate 40, to the left. The drive plate 44 is balanced hydraulically by suitable dimensioning of the ports 56, 56' and the sealing gaps 55, 57 between the drive plate and the cover 42b of the housing in a manner whereby the axial forces exerted on this side by the working fluid on the drive plate 44 counteract and as near as possible coincide, with respect to their total magnitude with the axial forces from the piston-cylinder assemblies on the other side of the plate 44.

Hydraulic balancing can be accomplished in an analogous manner on the opposite side of the axial force system, at guide plate 40. Pressure fluid can be passed through passages 21 in the seating bodies 20, with their extensions on pins 23, to cavities 70, located in the side of the guide plate 40 abutting the cover member 42a, the cavities being so dimensioned that axial hydraulic balance is obtained in relation to the forces exerted by the ring of piston-cylinder assemblies.

It should be particularly noted that it is possible with the bent-axis machine of the present invention to provide in a simple manner hydraulic balance of the axial bearing forces on both sides of the load generating piston-cylinder system. With conventional bent-axis machines, the axial reaction forces acting from the pistons on the drive plate can only be balanced hydraulically with great difficulty and must be provided with axial force absorbing roller bearings or the like, which results in increased weight and restricted useful life of the machine.

Further, as illustrated in FIG. 1, and as disclosed in my copending application Ser. No. 114,935, the contact surface between each cylinder and piston and its respective seat is preferably provided close to a great circle on. the seat, with the contact surface having a diameter of approximately the same magnitude as or slightly smaller than the maximum cross-sectional pressure area of the cylinder-piston unit.

The foregoing description is concerned with a positive displacement type bent-axis machine constructed in accordance with the invention. The principles applied, however, enable a variable displacement angle piston machine to be constructed, this type of machine having long been desired in the field and for many applications necessary in modern hydraulic technology. By making relatively small changes, the machine illustrated in FIG. 1 and 2 can be converted to a variable displacement machine, as hereinafter described. I

FIGS. 3 and 4 illustrate an embodiment of a variable displacement hydraulic angle piston machine constructed in accordance with the invention, the pistoncylinder assemblies and surrounding components substantially coinciding with the corresponding components in the aforedescribed positive displacement machine. Thus, the pistons 112 and cylinders 110 of the assembly engage spherical seating surfaces 124 and 122 resp. arranged on seating bodies associated with a guide plate 140 and a guide plate 144 resp. Similarly to the aforedescribed embodiment, the guide plate is fix-' edly attached to a central pin 132 which serves to suppin 132 is not journalled directly to the surrounding housing, but is journalled in a slide-like, movable body 160, hereinafter called the slide. The side of the slide facing inwardly of the housing presents a planar surface which co-acts with the guide surface 140, while on its opposite side the slidehas a part cylindrical slide surface which co-acts with a complimentary shaped, concave cylindrical slide surface 162 located on the inside of the left end-wall portion 142 of the machine housing according to FIG. 3. The planar form of the slide is rectangular or square and its mutually parallel planar side surfaces 164, 166 extending perpendicular to the axis of the cylindrical slide surface co-act when sliding, with opposing surfaces in the end portion 142 of the housing. Thus, the slide can be likened to a very wide so-called Woodruff wedge accomodated in its cooperating groove. The center of the slide surface 162 coincides with the intersecting point 153 between the axis of rotation 152 and 154 of the guide plate 140 and the drive plate 144, respectively. Consequently, if the slide is moved along the slide surface 162 in a-direction parallel with the plane of the drawing the anglea between the axis of rotation will be changed while retaining the positionof the intersection point 153,

' side of the machine while the inclined cam or guide which causes a change in the displacement of the machine.

Movement of the slide can be caused in a number of different ways, both purely manually and by means of hydraulic servo devices for example. An example of hydraulic operation is illustrated in FIG. 4. A pin 168 projects from one, 164, of the parallel planar sides of the slide 160 out through an opening 170 in the opposing wall of the housing. The pin 160 engages in a groove in a servo-piston 172, which is slidably accomodated in a cylindrical bore 174 in the'housing parallel to the slide surface 162. By passing pressure fluid to and from the respective chambers 174'and 174 on opposite sides of the piston, the piston can be moved longitudinally, thereby moving the slide 160 to the desired position. The servo-piston 170 may naturally be arranged in a different manner relative to the slide 160, and similarly several pistons may be used for operating the slide, for example in a manner whereby one servo device identical with that described above is also arranged on the opposite side of the slide parallel with its surface'166, so that the slide is actuated by symmetrically engaging operating forces.

Telescopic piston-cylinder assemblies of the type preferably used in the bent-axis piston machine of the present invention have a number of principle advantages compared with conventional piston arrangements, as described in more detail in the aforementioned patent. Thus, such force transmitting elements as piston rods or piston shoes are eliminated as is also the frictional losses which occur when transmitting the piston forces. An important characteristic of the piston-cylinder assembly of the present invention is that no obliquely acting forces or tilting forces act between the pistons and cylinders, and that the common slide surface of the pistons and cylinders may be made short in relation to piston diameter and length of stroke, which enables shorter axial constructional lengths. I

Advantages in addition to thosementioned above are obtained when such telescopic piston-cylinder assemblies are used in bent-axis piston machine constructed in accordance with the invention. Commutation, i.e. guiding of the in-flow and out-flow of the working fluid to and from the assembly can be placed on the driving plate on the opposite end .of the machine becomes a unit which is independent of the hydraulic system. In turn, this means that the displacement of the machine can be easily varied by moving the guide plate in the aforedescribed manner. Moreover, this enables the embodiment of theaforedescribed combined driving and dead-space filling arrangement. The pins of this latter arrangement which protrude into the piston-cylinder assembly completely re'place other, less robust devices adapted for driving the guide plate, for example the universal joints used in known 'rnachines, as before mentioned.

Finally, particular attention should be paid to theproposed means for holding the sleeve-shaped pistons cylinder assembly. It should be noted that these springs are dynamically loaded, with accompanying risks, and that all of the springs are replaced by one single, statically loaded springs Moreover, since with the embodiment of the invention no space is required to house the springs, the piston-cylinder assemblies can be placed nearer the axis of rotation, and finally the difficulties caused by the centrifugal forces on the individual springs are eliminated. Generally speaking, with the machine of the present invention the mass forces caused by centrifugal forces have only a slight restricting effect on the speed of the machine, since the components of the rotary system most sensitive to these forces, namely the pistons and cylinders of the pistoncylinder assembly, are well supported at the ends of their seating bodies and-are capable of absorbing relatively large radial forces, and because of their sleeveshaped configuration are particularly light. The interior of the sleeve-shaped pistons and cylinders is substantially filled by the pins serving as the combined drive means and dead-room filling means, which pins together with the seating bodies and the guide plate form a separate statically and dynamically balanced system.

In summary it can be said that the hydraulic machine of the present invention presents, if not a vhybn'de machine, nevertheless a hydraulic piston machine which constitutes an intermediate form of the two main types of known machines of the type in question, the inline machines and the bent-axis machine. It combines the compactness and structural favorability of the inline machine, particularly for providing variable displacement, and eliminates axial bearing forces, with the advantages of bent-axis-rnachine with respect to the function of the system with but small deviation between the directions of the primary hydraulic piston forces and the axis directions of the pistons, and the small dead space in the cylinders.

Although the invention has been described with reference to a number of embodiments thereof it will be understood that the invention is not restricted thereto but can be modified within the scope of the following claims.

I claim:

1. In a fluid pressure axial piston machine of the bent-axis type having a housing, a plurality of piston-v cylinder units positioned in the housing and disposed in substantially uniform and spaced relationship around the substantially parallel with a central axis, each piston-cylinder unit including a tubular cylinder having a tubular piston slideably disposed therein, a shaft rotatably supported on said housing and having it axis intersecting said central axis at an angle, first and second rotatable support bodies disposed adjacent the opposite axial ends of said'plurality of cylinder-piston units, said first support body being rotatably supported on said housing for rotation by said central axis, said second support body being coaxial with and connected to said shaft for rotation therewith, a plurality of pairs of first and second reaction-force absorbing seats mounted on said first and second support bodies respectively, said seats having a partial spherical convex surface thereon, each of said cylinder-piston units being movably and detachably positioned between a pair of said first and second seats, means for causing synchronous rotation of said first and second support bodies, and passage means for permitting working fluid in correct phase sequence to pass to and from the interior of the respective cylinder-piston units, comprising the improvement wherein a holding device common to all piston-cylinder units coacts between the plurality of pistons and the plurality of cylinders for resiliently urging each cylinder-piston unit toward an expanded position for maintaining the ends of the pistons and cylinders in resilient engagement with their respective seats, each piston having an open end resiliently maintained in engagement with the spherical surface of one of said first seats, and each cylinder having an open end resiliently maintained with the spherical surface of one of said second seats, the contact surface between each cylinder and piston and its respective seat being provided close to a great circle on the seat, and the contact surface having a diameter of approximately the same magnitude as the maximum cross-sectional pressure area of the cylinder-piston unit.

2. A machine according to claim 1, further including slide means movably mounting said first support body on said housing for movement of said first support body relative to said housing along a path substantially parallel to a plane containing therein said central axis and the axis ofsaid shaft, the slide means being provided with a convex cylindrical slide surface thereon disposed in slidable engagement with a corresponding concave cylindrical slide surface formed on the housing, the center of rotation of the slide surfaces being located substantially at the intersection point between the central axis and the axis of the shaft, whereby movement of the slide means relative to the housing varies the angle between the axis of the shaft and the central axis.

3. A machineaccording to claim 1, wherein the contact surface between each cylinder and piston and its respective seat lies somewhat within the maximum crosssectional pressure area of the cylinder-piston unit.

4. A machine according to claim 1, wherein the holding device includes a disc arranged substantially coaxially with the ring of piston-cylinder units and engaging the cylinders thereof, a retainer member positioned opposite said disc, said retainer member being substantially coaxial with the ring of piston-cylinder units and engaging the pistons thereof, and spring means positioned centrally of the ring of cylinder-piston units and between the disc and the retainer member to urge said disc and retainer member away from one another to maintain the cylinders and pistons in engagement with their respective seats.

5. A machine according to claim 4, wherein the first rotatable support body has a central pin mounted thereon, said holding device including a seat member slideably supported on the central pin, and said spring means comprising a coil spring disposed in sliding relationship to the central pin and having one end thereof positioned in engagement with the seat member, the seat member being resiliently urged by the spring into engagement with one of the disc and retainer member and having a curved surface to permit said one of said disc and retainer member to rollingly bear thereagainst, the other end of the coil spring being cooperatively engaged with the other of the disc and retainer member.

6. A machine according to claim 1, wherein the pistons are maintained in engagement with the seats secured to the first rotatable support body and the cylinders are maintained in engagement with the seats secured to the second rotatable support body, and the means for causing synchronous rotation of the first and second support bodies including extensions mounted on the seats secured to the first support body, said extensions being in the form of pins which project into the interior of the pistons and engage the inside surfaces of the pistons by means of a line-like contact during rotation of the machine so that the torque exerted on the first rotatable support body is transmitted through the cylinder-piston units to the second rotatable support body.

7. A machine according to claim 6, wherein the pins substantially fill the interior :space of the pistoncylinder units when the individual units are in a contracted position.

8. A machine according to claim 7, wherein the holding device includes a disc arranged substantially coaxially with the ring of piston-cylinder units and engaging the cylinders thereof, a retainer member positioned o'pposite said disc, said retainer member being substantially coaxial with the ring of piston-cylinder units and engaging the pistons thereof, and spring means positioned centrally of the ring of cylinder-piston units and between the disc and the retainer member to urge said disc and retainer member away from one another to maintain the cylinders and pistons in engagement with their respective seats. 

1. In a fluid pressure axial piston machine of the bent-axis type having a housing, a plurality of piston-cylinder units positioned in the housing and disposed in substantially uniform and spaced relationship around the substantially parallel with a central axis, each piston-cylinder unit including a tubular cylinder having a tubular piston slideably disposed therein, a shaft rotatably supported on said housing and having it axis intersecting said central axis at an angle, first and second rotatable support bodies disposed adjacent the opposite axial ends of said plurality of cylinder-piston units, said first support body being rotatably supported on said housing for rotation by said central axis, said second support body being coaxial with and connected to said shaft for rotation therewith, a plurality of pairs of first and second reaction-force absorbing seats mounted on said first and second support bodies respectively, said seats having a partial spherical convex surface thereon, each of said cylinder-piston units being movably and detachably positioned between a pair of said first and second seats, means for causing synchronous rotation of said first and second support bodies, and passage means for permitting working fluid in correct phase sequence to pass to and from the interior of the respective cylinder-piston units, comprising the improvement wherein a holding device common to all pistoncylinder units coacts between the plurality of pistons and the plurality of cylinders for resiliently urging each cylinderpiston unit toward an expanded position for maintaining the ends of the pistons and cylinders in resilient engagement with their respective seats, each piston having an open end resiliently maintained in engagement with the spherical surface of one of said first seats, and each cylinder having an open end resiliently maintained with the spherical surface of one of said second seats, the contact surface between each cylinder and piston and its respective seat being provided close to a great circle on the seat, and the contact surface having a diameter of approximately the same magnitude as the maximum cross-sectional pressure area of the cylinder-piston unit.
 2. A machine according to claim 1, further including slide means movably mounting said first support body on said housing for movement of said first support body relative to said housing along a path substantially parallel to a plane containing therein said central axis and the axis of said shaft, the slide means being provided with a convex cylindrical slide surface thereon disposed in slidable engagement with a corresponding concave cylindrical slide surface formed on the housing, the center of rotation of the slide surfaces being located substantially at the intersection point between the central axis and the axis of the shaft, whereby movement of the slide means relative to the housing varies the angle between the axis of the shaft and the central axis.
 3. A machine according to claim 1, wherein the contact surface between each cylinder and piston and its respective seat lies somewhat within the maximum cross-sectional pressure area of the cylinder-piston unit.
 4. A machine according to claim 1, wherein the holding device includes a disc arranged substantially coaxially with the ring of piston-cylinder units and engaging the cylinders thereof, a retainer member positioned opposite said disc, said retainer member being substantially coaxial with the ring of piston-cylinder units and engaging the pistons thereof, and spring means positioned centrally of the ring of cylinder-piston units and between the disc and the retainer member to urge said disc and retainer member away from one another to maintain the cylinders and pistons in engagement with their respective seats.
 5. A mAchine according to claim 4, wherein the first rotatable support body has a central pin mounted thereon, said holding device including a seat member slideably supported on the central pin, and said spring means comprising a coil spring disposed in sliding relationship to the central pin and having one end thereof positioned in engagement with the seat member, the seat member being resiliently urged by the spring into engagement with one of the disc and retainer member and having a curved surface to permit said one of said disc and retainer member to rollingly bear thereagainst, the other end of the coil spring being cooperatively engaged with the other of the disc and retainer member.
 6. A machine according to claim 1, wherein the pistons are maintained in engagement with the seats secured to the first rotatable support body and the cylinders are maintained in engagement with the seats secured to the second rotatable support body, and the means for causing synchronous rotation of the first and second support bodies including extensions mounted on the seats secured to the first support body, said extensions being in the form of pins which project into the interior of the pistons and engage the inside surfaces of the pistons by means of a line-like contact during rotation of the machine so that the torque exerted on the first rotatable support body is transmitted through the cylinder-piston units to the second rotatable support body.
 7. A machine according to claim 6, wherein the pins substantially fill the interior space of the piston-cylinder units when the individual units are in a contracted position.
 8. A machine according to claim 7, wherein the holding device includes a disc arranged substantially coaxially with the ring of piston-cylinder units and engaging the cylinders thereof, a retainer member positioned opposite said disc, said retainer member being substantially coaxial with the ring of piston-cylinder units and engaging the pistons thereof, and spring means positioned centrally of the ring of cylinder-piston units and between the disc and the retainer member to urge said disc and retainer member away from one another to maintain the cylinders and pistons in engagement with their respective seats. 